The Fischer-Tropsch process is a well-documented process used for the conversion of synthesis gas (from hydrocarbonaceous feedstocks) into liquid and/or solid hydrocarbons. Generally, the feedstock (e.g. natural gas, associated gas and/or coal bed methane, heavy and/or residual oil fractions, coal, biomass) is converted in a first step into a mixture of hydrogen and carbon monoxide, often referred to as synthesis gas or syngas. The synthesis gas is then fed into one or more reactors where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into mainly paraffinic compounds ranging from methane to high molecular weight modules comprising up to 200 carbon atoms, though even higher paraffins can be produced under certain conditions.
In the Fischer-Tropsch process, it is desirable to maximise the amount of C5+ hydrocarbons produced and minimise production of methane. C5+ selectivity can be influenced by a number of factors, including the choice of catalyst.
Typically, Fischer-Tropsch catalysts are based upon the transition metals, particularly cobalt, iron and ruthenium, of which cobalt-based catalysts are known to be highly active and are especially favoured when the feedstock is natural gas. Cobalt-based Fischer-Tropsch catalysts are also favoured for use in low temperature processes.
Most commonly, cobalt is dispersed on a solid porous support material. A solid porous support material may also be referred to as carrier. The carrier may, for example, be a refractory metal oxide support, such as alumina, silica, titania and, to a lesser extent, zirconia. By application of cobalt, or a cobalt comprising compound, on a carrier a catalyst is obtained. One method of applying cobalt, or a cobalt comprising compound, is by means of impregnation.
Various methods of impregnating a solid porous support material with a metal are known; melt impregnation and incipient wetness impregnation being two such examples.
Before use in a Fischer-Tropsch process, activation of the catalyst is generally required, which activation may involve one or more steps. Activation generally includes a reduction step in which a stream of hydrogen gas is passed over the catalyst at elevated temperature.
There is a need for new and improved silica-comprising Fischer-Tropsch catalysts and for processes for preparing such catalysts. Especially silica comprising catalysts with a decreased methane selectivity and an enhanced C5+ selectivity in Fischer-Tropsch processes are desired.